©O41.3135 


tt Sa . The 
Electric Power Club 
Instructions | 
For Care and Operation 
of Transformers 





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The Electric Power Club 


Instructions 
For Care and Operation 
of Transformers 


Part 
Power Transformers 


fecevere ILE 
Distribution Transformers 


Fourth Edition 
April—1926 


Copyright, 1926, by 
THE ELECTRIC POWER CLUB 
These Instructions may be copied 
without change, due credit being 
given to The Electric Power Club. 


The Electric Power Club 
B. F. Keith Building 
Cleveland, Ohio, U.S. A. 





Price 25 cents Printed in U. S. A. 


Foreword 


manufacturers of electric power apparatus and control 

equipment, first organized in 1908 as The American Asso- 
ciation of Electric Motor Manufacturers, for the improvement in 
design and construction, the standardization, and the increased 
distribution of such products. 


6 Petre POWER CLUB “isan association» o 


The standards and practices adopted by The Electric Power 
Club during the past eighteen years are published in The Electric 
Power Club Handbook of Standards—Electric Power Apparatus. 
The transformer standards are also published in a separate publi- 
cation by the Transformer Section of the Club, which also issues 
this book of instructions for the care and operation of both 
power and distribution transformers. 


The member companies composing the Transformer Section of 
The Electric Power Club are listed on the inside back cover. 
These companies manufacture the great preponderance of the 
distribution and power transformers produced in the United 
States. 


This booklet is a reference work of practical information con- 
taining instructions for the proper care and operation of both 
power and distribution transformers, and is supplementary to 
the standards of manufacture, performance and test published in 
the book of transformer standards. Representing, as it does, the 
recommendations of the Transformer Section of The Electric 
Power Club, this booklet is found most useful by electrical engi- 
neers, superintendents of distribution, line foremen, electricians, 
operators, students and others responsible for the operation and 
maintenance of transformers. 


Standards applying to the manufacture, performance and test 
of other electric power apparatus and control equipment, also 
rules for their proper installation, operation and care, are given 
in other Electric Power Club publications, a list of which appears 
on the inside front cover. Information regarding standards and 
practices not covered in any of the Club’s publications may be 
secured by addressing The Electric Power Club, B. F. Keith 
Building, Cleveland, Ohio, U.S. A. 


E. P. C. 2 April, 1926 


PART I 


of Power Transformers 
(All sizes above 200 Kv-a.) 


HE successful operation of transformers is de- 

pendent upon proper installation and operation 
as well as upon proper design and manufacture. 
While a transformer requires less care than almost 
any other type of electrical apparatus, neglect of 
certain fundamental requirements may lead to 
serious trouble, if not to the loss of the transformer. 
For this reason, a wide distribution of information 
in regard to the proper care of transformers is im- 
portant, and these brief instructions are published 
for that purpose. 


Instruction for Care and Operation 


April, 1926 


Instructions for Care and Operation 
of Power Transformers 


These instructions apply to all power transformers and to all distribution 
transformers above 100 Kv-a. 1n capacity or for operation 
on 22000 volts or higher. 


Location 


Accessibility, ventilation and ease of in- 
spection should be given careful considera- 
tion in locating transformers. 


Water-Cooled Transformers depend 
almost entirely upon the flow of water 
through the cooling coils for carrying away 
heat, so that the temperature of the sur- 
rounding air has little effect upon that of 
the transformers. For this reason air cir- 
culation is of minor importance and water- 
cooled transformers may be located in any 
convenient place without regard to ventila- 
tion. 


Self-Cooled Transformers depend en- 
tirely upon the surrounding air for carry- 
ing away their heat. For this reason care 
must be taken to provide adequate ventila- 
tion. For indoor installation the room in 
which the transformers are placed must be 
well ventilated so that heated air can escape 
readily and be replaced by cool air from 
outside. 
lets required will depend on their distance 
above the transformer, and on the efficiency 
and load cycle of the apparatus. In general, 
about 20 sq. ft. of opening should be pro- 
vided for each 1000 Kv-a. of transformer 
capacity. Air inlets should be provided with 
the same total area as the outlets. If the 
transformer will be required to operate for 
considerable periods at continuous full load, 
the areas of the openings should be in- 
creased to about 40 sq. ft. per 1000 Kv-a. of 
transformer capacity. Self-cooled trans- 
formers of the indoor type must be so lo- 
cated that water cannot fall on the tanks 
or rain blow upon them. 


Self-Cooled Transformers should al- 
ways be separated from one another and 
from adjacent walls, partitions, etc., in 
order to permit free circulation about the 
tanks. This separation should not be less 
than 24 in. to 36 in., depending on size. 


Storage 


When i transformer can be set. ups im- 
mediately in its permanent location and 


EP. ¢. 


The number and size of air out-. 


filled with oil, it is advisable to do so, even 
though it will not be put into service for 
some time. If this is not convenient it 
should be stored in a dry place, having no 
rapid or radical temperature changes, and 
if possible, immersed in dry transformer oil. 
The transformer should not be stored or 
operated in the presence of corrosive gases 
such as chlorine, etc. If an indoor trans- 
former is stored OUTDOORS, it should be 
thoroughly covered to keep out rain. 


Handling 


When lifting a transformer, the lifting 
cables must be held apart by a spreader to 
avoid bending the lifting studs or other 
parts of the structure. 


Where a transformer cannot be handled 
by a crane, it may be skidded or moved on 
rollers, but care must be taken not to dam- 
age the base, or tip it over. A transformer 
should never be lifted or moved by placing 
jacks or tackle under the drain valve, cool- 
ing coil outlets, radiator connections or 
other attachments. When rollers are used 
under large transformers, skids must be 
used to distribute the stress over the base. 


When working about a transformer par- 
ticular care must be taken in handling all 
tools and other loose articles, since any- 
thing metallic dropped among the windings 
and allowed to remain there may cause a 
breakdown. 


Installation 


Transformers are in first class operating 
condition when shipped by the manu- 
facturer, i. e., they have been thoroughly 
tested for defects and are perfectly dry. 


When received, examination should be 
made before removing from cars and if any 
injury is evident or any indication of rough 
handling is visible, railroad claim should be 
filed at once and the manufacturer notified. 


Moisture may condense on any metal if 
the metal is colder than the air, and if 
present, it lowers the dielectric strength 


April, 1926 


POWER TRANSFORMERS 





and may cause a failure of the trans- 
former. Therefore, if transformers or 
oil drums are brought into a room warm- 
er than they are, they should be allowed 
to stand before opening until there is no 
condensation on the outside and they are 
thoroughly dry. 


Before being set up, a transformer should 
be inspected for breakage, injury or mis- 
placement of parts during shipment, and 
thoroughly examined for moisture. In addi- 
tion all accessible nuts, bolts and _ studs 
should be tightened. 


If transformers are water cooled, the 
cooling coils should be tested for leaks 
at a pressure of 80 to 100 lbs. per sq. in. 
Water, oil or preferably air, may be used 
in the coil for obtaining the pressure. 
The coil must be outside the tank, i. e., away 
from the coil insulation if water is used for 
the pressure test. When pressure is ob- 
tained, the supply should be disconnected and 
after 1 hour it should be determined whether 
any fall in pressure is due to a leak in the 
coil, or is in the fittings at the ends of the 
coil. 


Transformers Shipped Filled With 
Oil 

Each transformer shipped filled with oil 

should be inspected to see whether there is 


any condition indicating the entrance of 
moisture during shipment. 


If the transformer is received in dam- 
aged condition, so that water or other for- 
eign material has had a chance to enter the 
tank, the transformer should be emptied of 
oil and treated as though not shipped in oil, 
and in no case may drying be omitted. 


In all cases samples of oil should be taken 
from the bottom and tested. The dielectric 
strength of the oil when shipped is at least 
22 Kv. between 1 in. disks spaced 0.1 in. 
apart. A new transformer should not be 
put into service with oil which tests below 
this value. 


Transformers Shipped Assembled 
Without Oil 


Each transformer shipped assembled but 
not filled with oil should be carefully in- 
spected for damage in shipment. A thorough 
inspection can only be made by removing 
core and coils from the tank. All dirt 
should be wiped off and parts examined for 
breakage or other injuries. All conductors 
and terminals should be examined to check 
their proper condition and position. The 


Ere. ©. 


coil and core clamps should be tightened 
if necessary. 


The tank should be inspected and, if nec- 
essary, cleaned. 


When a transformer is shipped assembled 
but not filted with oil, moisture may be ab- 
sorbed during transportation. For this rea- 
son it is good practice to dry out all such 
transformers, especially transformers above 
7500 volts, before being put into service. 


Transformers Shipped Disassembled 


Only very large transformers are shipped 
in this way, and special instructions cover- 
ing features incident to this method of 
shipping are supplied by the manufacturer. 
These instructions should be carefully fol- 
lowed. 


Drying Core and Coils 


There are a number of approved methods 
of drying out transformer core and coils, 
any one of which will be satisfactory if 
carefully performed. However, too much 
stress cannot be laid upon the fact that if 
carelessly or improperly performed, great 
damage may result to the transformer in- 
sulation through over-heating. 


The methods in use may be broadly di- 
vided into two classes: 
1. Drying with the core and coils in the 
tank with oil. 
2) Oryine with ther oil cemoved,. cine 
core and coils may or may not be 
removed from the tank. 


Under the first class, the moisture is 
driven off by sending current through the 
winding while immersed in oil, with the top 
of the tank open to the air, or some other 
arrangement made for adequate ventilation. 
This may be done by: 

(a) The short circuit method (to be used if 
the transformer is new or has been out 
of service without oil any length of 
time). 

(b) The normal operation method (to be 
used if the transformer is already in 
service but shows moisture condensa- 
tion and the transformer cannot be shut 
down to apply the short circuit 
method). 


Short Circuit Method With Oil 


This method consists in heating the wind- 
ings and oil up to a high temperature for 
a limited time under short circuit with a 
partial load on the windings, the high oil 
temperature being obtained by blanketing 


April, 1926 


CARE AND OPERATION 


the tank (or reducing the flow of water 
for water-cooled transformers). When a 
transformer is short circuited in this 
manner only a fraction of the normal 
voltage should be applied to one wind- 
ing. In using this method, if the load 
does not exceed one-half or three- 
quarters full load, the winding tempera- 
ture is not greatly in excess of the maximum 
or top oil temperature, so that high oil tem- 
peratures are safe without resulting in any 
deterioration of the insulation from local 
over-heating. With good ventilation, the 
moisture, as it is driven off in the form of 
vapor will escape to the outside atmosphere 
and no condensation of moisture will take 
place on the underside of the cover or else- 
where in the tank. 


The following table shows the short cir- 
cuit current in per cent of full load current 
which may be used for this method of dry- 
ing transformers, with the corresponding 
maximum allowable top oil temperature in 
degrees centigrade. Less than 5 per cent 
of normal voltage will usually be required to 
circulate the current in the windings. 


Short Circuit Amps. in 


Percent of full Load Maximum 


Self-Cooled Water-Cooled Top Oil 
Transformers Transformers Temperature 
50 50 85 deg. C. 
AS 60 80 deg. C. 
85 7 75 deg. C. 


These temperature limits and loads must 
be strictly adhered to in order to obtain the 
desired results without danger to the trans- 
formers. 


It should be noted that the higher allow- 
able temperatures go with the smaller loads, 
that is, more blanketing or less water will be 
required for the smaller loads than for the 
higher, in order to bring the oil temperature 
up to the point shown in the table. 


Ventilation—During the drying run, 
additional ventilation to that ordinarily pro- 
vided should be maintained by slightly rais- 
ing the manhole cover and protecting the 
opening from the weather. The entire cover 
should be lagged with heat insulating mate- 
rial to prevent condensation of moisture 
within. 

When to Discontinue Drying—. Drying 
should be continued until oil from the top 
and bottom of the tank tests 22 Kv. or 
higher between 1.0 in. square edge discs 
spaced 0.1 in. for seven consecutive tests 
taken 4 hrs. apart with the oil maintained 
at maximum temperature for the load held 
and without filtering. All ventilating open- 
ings should then be closed and the trans- 
former kept at the same temperature for an- 


BUP,C: 


other 24 hrs. without filtering the oil and as 
before the oil should be tested at 4 hr. inter- 
vals. A decrease in the dielectric strength 
of the oil indicates that moisture is still 
passing from the transformer into the oil 
and drying should be continued. 

Unless constant or increasing dielectric 
strength as shown by these tests indicates 
that drying is completed, the ventilators 
should be opened, the oil filtered and the 
drying process continued. 

After the short circuit run is discon- 
tinued, the transformer should be operated 
for 24 hrs. at approximately two-thirds 
voltage and at the same high temperature, 
making similar tests of oil samples and 
filtering the oil if necessary. After satis- 
factory two-thirds voltage test, full voltage 
should be. applied for 24 hrs. and the same 
tests repeated. Water-cooled transformers 
may require some water to hold the top oil 
temperature within the 85 deg. C. limit 
during this test. 


Normal Operation Method 


This method consists in providing ventila- 
tion in the cover, as explained above, and 
blowing air across the top oil, say by a desk 
fan, while the transformer is under normal 
operation. 

The manhole cover should be inspected 
frequently and the oil temperature raised by 
blanketing the tank or increasing the load. 
If condensation appears on the under side of 
the manhole cover, the oil temperature 
should be reduced and the run continued 
until the moisture disappears, when the tem- 
perature should again be raised. 


The following table shows the highest 
permissible load current for maximum top 
oil temperatures of 70, 75 and 85 deg. C.: 


Load Amps. in 


Percent of full Load Maximum 
Self-Cooled Water-Cooled Top Oil 
Transformers Transformers Temperature 
50 50 85 deg. C. 
85 75 75 degC: 
100 85 70 deg. C. 


The operation should be continued for 3 
days at maximum temperature; then if no 
condensation occurs, the drying may be dis- 
continued as outlined above in the paragraph 
“When to Discontinue Drying”. 

Typical of the second class, i. e., drying 
with the oil removed, are the three following 
methods: 

A—By internal heat. 
B—By external heat. 
C—By internal and external heat. 


(A) By Internal Heat—For this meth- 
od alternating current is required. The 


April, 1926 


ry 


POWER TRANSFORMERS 





transformer should be placed in its tank 
without the oil and the cover left off to 
allow free circulation of air. Either wind- 
ing can be short circuited and sufficient 
voltage impressed across the other winding 
to circulate enough current through the coils 
to maintain the temperature at from 75 deg. 
C. to 80 deg. C. About one-fifth of normal 
full-rated current is generally sufficient to do 
this. Ihe impressed voltage necessary to 
circulate this current varies within wide 
limits among different transformers, but will 
generally be approximately ™% of 1 per cent 
to 1% per cent of normal voltage, at normal 
frequency. 

The end terminals of the winding must be 
used, not taps, so that current will circulate 
through the total winding. The amount of 
current may be controlled by a rheostat in 
series with the exciting winding. 

This method of drying out is superficial 
and slow and should only be used with small 
transformers, and then only when local con- 
ditions prohibit the use of one of the other 
methods. 

(B) By External Heat—The trans- 
former should be placed in a wooden box 
with holes in the top and near the bottom 
to allow air circulation. The clearance be- 
tween the sides of the transformer and the 


-box should be small so that most of the 


heated air will pass up through the ventilat- 
ing ducts among the coils and not around 
the sides. The heat should be applied at the 
bottom of the box. 


The best way to obtain the heat is from 
erid resistors, using either alternating or 
direct current. The temperature limits are 
the same as for Method A. The trans- 
former must be carefully protected against 
direct radiation from the ‘heaters. Care 
must also be taken to see that there is no 
inflammable material near the heaters, and 
to this end it is advisable to completely line 
the wooden box with asbestos. 


Instead of placing the heater inside the 
box containing the transformer, it may be 
placed outside and the heat carried into the 
bottom of the box through a suitable pipe. 
Where this plan is followed, the heat may 
be generated by the direct combustion of 
gas, coal or wood, provided that none of 
the products of combustion be allowed to 
enter the box containing the transformer. 
Heating by combustion is not advocated ex- 
cept when electric current is not available. 

This method, while effective, requires a 
much longer time than Method C. 

(C) By Internal and External Heat— 
This is a combination of Methods A and B. 
The transformer should be placed in a box 
and external heat applied as in B and cur- 


EVE.C 


rent circulated through the windings as in 
A. The current should, of course, be con- 
siderably less than when no external heat is 
applied. 

This method is used occasionally where 
direct current only is available, a. certain 
amount of current being passed through the 
high voltage winding only, as the cross sec- 
tional area of the low voltage conductor is 
generally too large for it to be heated with 
an economical amount of direct current. 
The use of direct current for drying out is 
not recommended except where alternating 
current cannot be obtained. When this 
method of drying is used, the temperature 
should be measured by the increase in re- 
sistance method. 


Method C is quicker than either A or B 
and has the great advantage that the insula- 
tion is heated much more uniformly. 


Time Required for Drying—There is 
no definite length of time for drying. One 
to 3 weeks will generally be required, de- 
pending upon the condition of the trans- 
former, the size, the voltage and the method 
of drying used. 


Insulation Resistance—The measure- 
ment or determination of insulation re- 
sistance is of value in determining the course 
of drying, only when the transformer is 
without oil. If the initial’ insulation § re- 
sistance be measured at ordinary tempera- 
tures, it may be high although the insulation 
is not dry, but as the transformer is heated 
up, it will drop rapidly. 

As the drying proceeds at a constant tem- 
perature, the insulation resistance will gen- 
erally increase gradually until towards the 
end of the drying period when the increase 
will become more rapid. Sometimes the re- 
sistance will rise and fall through a short 
range one or more times before reaching a 
steady high point. This is caused by mois- 
ture in the interior parts of the insulation 
working its way out through the outer por- 
tions which were dried at first. 


As the temperature varies, the insulation 
resistance also varies greatly, therefore the 
temperature should be kept nearly constant 
and the resistance measurements should all 
be taken at as nearly the same temperature 
as possible. The insulation resistance in 
megohms varies inversely with the tempera- 
ture and for a 10 deg. C. change of tempera- 
ture, the megohms change by a ratio of 2:1. 
Measurements should be taken every 2 hours 
during the drying period. 


Resistance Curve—A curve of the in- 
sulation resistance measurements should be 
plotted with time as abscissae and resistance 
as ordinates. By observation, the knee of the 


April, 1926 


CARE AND OPERATION 





curve (i. e., the point where the insulation 
resistance begins to increase more rapidly) 
can be determined and the run should con- 
tinue until the resistance is constant for 12 
hours. 


Precautions to be Observed in Drying 
Without Oil—As the drying temperature 
approaches the point where fibrous materials 
deteriorate, great care must be taken to see 
that there are no points where the tempera- 
ture exceeds 85 deg. C. Several thermome- 
ters should be used and they should be placed 
well in among the coils near the top and 
screened from air currents. Ventilating 
ducts offer particularly good places in which 
to place some of the thermometers. As the 
temperature rises rapidly at first, the ther- 
mometers must be read at intervals of about 
%4 hour. In order to keep the transformer 
at a constant temperature for insulation re- 
sistance measurements, 1 thermometer should 
be placed where it can be read without re- 
moving it or changing its position. The 
other thermometers should be shifted about 
until the hottest points are found, and should 
remain at these points throughout the drying 
period. Wherever possible, the temperature 
should be checked by the increase in re- 
sistance method. 


Caution—It is well to have a chemical 
fire extinguished or a supply of sand at hand 
for use in case of necessity. 


It is not safe to attempt the drying out of 
transformers without giving them constant 
attention. 


Sampling and Testing of Oil 


The sample container should be a large 
mouth glass bottle. All bottles should be 
cleaned and dried with gasoline before be- 
ing used. A cork stopper should be used. 


The sample for dielectric tests should be 
at least 16 oz., and if other tests are to be 
made, 1 quart (32 oz.). 


Test samples should be taken only after 
the oil has settled for some time, varying 
from 8 hours for a barrel to several 
days for a large transformer. Cold oil is 
much slower in settling and may hardly set- 
tle at all. Oil samples from the transformer 
should be taken from the oil sampling valve 
at the bottom of the tank. Oil samples 
from a barrel should be taken from the 
bottom of the drum. A brass or glass 
“thief” can be‘conveniently used for this 
purpose. The same method should be used 
for cleaning the “thief” as is used for clean- 
ing the container. 


When drawing samples of oil from the 
bottom of the transformer or large tank, 
sufficient oil must first be drawn off to make 


Bae Cc 


sure that the sample will be comprised of 
oil from the bottom of the container, and 
not from the oil stored in the sampling pipe. 
A glass receptacle is desirable so that if 
water is present, it may be readily observed. 
If water is found, an investigation of the 
cause should be made and a remedy applied. 
If water is not present in sufficient quan- 
tity to settle out, the oil may still contain 
considerable moisture in a suspended state. 
It should, therefore, be tested for dielectric 
strength. 


Testing 


For testing oil for dielectric strength some 
standard device for oil testing should be 
used. The standard oil testing spark gap 
has disc terminals 1 in. in diameter spaced 
0.1 in. apart. The testing cup should be 
cleaned thoroughly to remove any particles 
of cotton fibre, and rinsed out with a por- 
tion of the oil to be tested. 


The spark gap receptacle should be 
nearly filled with oil and allowed to stand 
for a few minutes to give bubbles time to 
escape before making the test. 


The rate of increase in voltage should be 
about 3000 volts per second. Five break- 
downs should be made on each filling and 
then the receptacle emptied and refilled with 
fresh oil from the original sample. The 
average voltage of 15 tests (5 tests on each 
of 3 fillings) is usually taken as the di- 
electric strength of the oil. It is recom- 
mended that the test be continued until the 
mean of the averages of at least 3 fillings is 
consistent. 


The dielectric strength of oil when 
shipped is at least 22 Ky. tested in the 
standard gap. If the dielectric strength of 
the oil in a transformer in service, tests at 
less than 16,500 volts, it should be filtered. 
New oil of less than the standard dielec- 
tric strength should not be put in a trans- 
former. 


Drying Oil and Filling 
Transformer 


In removing moisture from transformer 
oil, it is preferable to filter from one tank 
and discharge into another, although if nec- 
essary it may be drawn from the bottom of 
a tank and discharged at the top. When 
there is much water in the oil, it should be 
allowed to settle, then drawn off and treated 
separately. 


Filling Transformers 


Before the transformer is filled with oil 
all accessories, such as valves, gauges, ther- 
mometers, plugs, etc., must be fitted to the 


April, 1926 


POWER TRANSFORMERS 





transformer and made oil tight. The 
threads should be filled with shellac before 
putting them in place. The transformers 
must be thoroughly cleaned. 


Metal hose must be used instead of rub- 
ber hose, because oil dissolves the sulphur 
found in rubber, and may cause trouble by 
the sulphur attacking the copper. 


The oil used should be clean, dry oil of 
the grade recommended by the manu- 
facturer. 


The use of a filter press is recom- 
mended and if one is not available some 
precaution should be taken to strain the 
oil before putting it in the transformer. 


After filling the transformer, the oil 
should be allowed to settle at least 12 hours 
and then samples taken from the bottom 
should be again tested before voltage is ap- 
plied to the transformer. 


It is very important that the surfaces of 
the oil when cold (25 deg. C.) be at the oil 
level indicated by the mark on the oil 
gauge. When the transformer is not in 
service, the oil level must never be allowed 
to fall to a point where it does not show 
in the gauge. When it is necessary to re- 
plenish the oil, care must be taken to see 
that no moisture finds its way into the tank. 
As the oil heats up with the transformer 
under load. it will expand and rise to a 
higher level. 


First Time in Service 


When the voltage is first applied to the 
transformer it should, if possible, be 
brought up slowly to its full value so that 
any wrong connection or other trouble may 
be discovered before damage results. After 
full voltage has been applied successfully, 
the transformer should preferably be oper- 
ated in that wav for a short period without 
load. It should be kept under observation 
during this time and also during the first 
few hours that it delivers load. After 4 or 5 
days’ service it is advisable to test the oil 
again for moisture. 


Water Circulation 


If the transformer is water-cooled, the 
main water valve should be opened as 
soon as the oil temperature reaches 45 
deg. C. If there are 2 or more sets of 
cooling coils in parallel. the valves of all 
sections should be adjusted for equal 
rates of flow. This can be estimated by 
feeling the weight of the discharge streams 
from the different sections. It can be de- 
termined best, however, by noting the dif- 
ference in temperature between ingoing and 
outgoing water from each section. A care- 
ful measure should be taken of the total 


EP. C, 


amount of water flowing through all sec- 
tions and the total rate of flow adjusted to 
that called for. 


Care 


The idea that a transformer in service 
needs no attention may lead to serious re- 
sults. Careful inspection is essential, and 
the directions given in this section should 
be followed. 


In spite of all precautions, moisture may 
be absorbed by the transformer; and dur- 
ing the first few days of operation it is well 
to inspect the inside of the manhole 
cover for moisture. If sufficient moisture 
has condensed ‘to drip from the cover, the 
transformer should be taken out of service 
and dried. The oil should be tested and 
dried if necessary. 


Oil 
Samples of oil from all transformers 


should be drawn and tested at least once 
every 6 months. 


During the first month of service of trans- 
formers having a potential of 40,000 volts 
or over, samples of oil should be drawn 
each week from the bottom of the tank and 
tested. 

If at any time the oil should test below 
16,500 volts, it should be filtered. 


Inspection 


No matter how satisfactory the opera- 
tion of a transformer may be, it should be 
taken out of service and thoroughly in- 
spected at least once a year. The inside 
of the cover and the tank above the oil 
should be regularly inspected to see that 
they are clean, dry and free from mois- 
ture and that the thermometer bulb is clean. 
If an appreciable amount of dirt or sedi- 
ment is found inside the case it is best to 
take out the transformer and remove the 
oil from the tank. The transformer and 
the tank should then he cleaned thoroughly 
and the oil filtered and tested. In cleaning, 
only dry cloths or waste should be used. 
Care should be taken to see that all 
nuts are tight and all parts in their proper 
places. If the transformer is water-cooled, 
the cooling coils should be cleaned thor- 
oughly. The transformer and the oil 
should be replaced in the tank and when 
the cover is put on, all cracks and open- 
ings closed tightly. 

In the case of water-cooled transformers, 
the rate of flow should be checked from 
time to time and if it is found to have di- 
minished the cause should be looked for 
and remedied. The most frequent cause of 
clogging of cooling coils is the presence of 


April, 1926 


CARE AND OPERATION 





air in the water, resulting in the formation 
of a scaly oxide. 


Removing Scale From Cooling Coils 


Scale and sediment can be removed from 
a cooling coil without removing the coil 
from the tank. Both inlet and outlet pipes 
should be disconnected from the water sys- 
tem and temporarily piped to a point a num- 
ber of feet away from the transformer, 
where the coil can be filled and emptied 
safely. Especial care must be taken to pre- 
vent any acid, dirt or water from getting 
into the transformer. 


All the water should be blown or 
syphoned from the cooling coils which 
should be then filled with a solution 
of hydrochloric (muriatic) acid, specific 
gravity 1.10. (Equal parts of commercially 
pure concentrated hydrochloric acid and 
water will give this specific gravity.) 

It may be found necessary to force this 
solution into the cooling coils. When this 
is done one end of the coil should be par- 
tially restricted, so that the solution will not 
be wasted when the coil is full. After the 
solution has stood in the coil about an hour, 
the coil should be flushed out thoroughly 
with clean water. If all the scale is not 
removed the first time, the operation 
should be repeated until the coil is clean, 
using new solution each time. The num- 
ber of times it is necessary to repeat the 
process will depend on the condition of the 
coil though ordinarily 1 or 2 fillings will be 
sufficient. 


As the chemical action which takes place 
may be very violent and may often force 
acid, sediment, etc., from both ends of the 
coil, it is well, therefore, to leave both ends 
partially open to prevent abnormal pressure. 


Idle Cooling Coils 


When a water-cooled transformer is idle 
and exposed to freezing temperatures the 
water must be blown out of the cooling coil. 
In addition to blowing out the water the 
cooling coils should be dried by forcing 
heated air through them. If not con- 
venient to do this the coil should be filled 
with transformer oil. 


Operation 


An artificially cooled transformer should 
not be run continuously, even at no-load, 
without the cooling medium. Therefore, it 
is essential to maintain a proper circula- 
tion in the cooling system. 


If the water circulation is stopped for 
any reason, the load should be immedi- 


LOM fel OF 


ately reduced as much as possible and 
close watch kept of the temperature of 
the transformer. When the oil at the top 
of the tank reaches 80 deg. C. the trans- 
former must be cut out of service at once. 
This temperature should be recognized as 
an absolute limit and must not be ex- 
ceeded. It should be held only during an 
emergency period of short duration. 


Nearly all cooling water will in time 
cause scale or sediment to form in the cool- 
ing coil. The time required to clog up the 
cooling coils depends on the nature and 
amount of foreign matter in the water. 
The clogging materially decreases the 
efficiency of the coil and is indicated by a 
high oil temperature and a decreased flow 
of water, load condition and water pres- 
sure remaining the same. 


Temperature 


Thermometers should be read daily or 
more often. If the indicated oil tempera- 
ture is 80 deg. C. or over for a self- 
cooled unit or 65 deg. C. or over for a 
water-cooled unit, the transformer must 
be cut out of service at once and the 
cause of the excessive heating investi- 
gated. Should a transformer remain in 
service any length of time under this con- 
dition, it may be seriously damaged. 


Regardless of oil temperature as indi- 
cated by thermometers, the transformer 
must not be operated at overloads not stipu- 
lated by the specifications or contract. When 
the contact specifies an overload, a water- 
cooled transformer operated at such over- 
load should have the amount of water in- 
creased in proportion to the load. On ac- 
count of the increased amount of water 
during overload the temperature of the 
oil will not rise as fast as the temperature 
of the windings and any of the causes lead- 
ing to excessive heating will have more pro- 
nounced effect under these conditions. 
Therefore, the transformer during overload 
should be watched with especial care to see 
that the oil temperature is kept well below 
the temperature limits specified. 


Moisture may get into a transformer 
due to the fact that as oil is heated and 
cooled it expands and contracts and 
therefore air is expelled from and enters 
the transformer. If the air which enters 
the transformer is at the same time 
cooled off by contact with cover to be- 
low its dew point moisture will condense. 


It is therefore good practice to operate 
transformers at several degrees above air 
temperatures at all times. This will large- 
ly prevent condensation. 


10 April, 1926 


ENP 


PART II 


of Distribution Transformers 
(Sizes of 200 Kv-a. and below) 


HE instructions for the care and operation of 

power transformers are somewhat too elaborate 
to be followed in connection with the operation of 
distribution transformers of small capacities which 
are used in very large quantities. The following 
simplified rules are suggested in connection with 
the care and operation of these smaller capacity dis- 
tribution transformers. 

Exceptions—It is recommended that for distribution 
transformers of voltages of 22000 volts and above and 
for capacities above 100 Kv-a., such for instance as are 
usually mounted on a platform or in other localities 
where they are readily accessible for inspection, the 
same rules be followed as are given in Part I for the 
care and operation of power transformers. 


11 


Instructions for Care and Operation 


April, 1926 


Instructions for Care and Operation 
of Distribution ‘Transformers 


Exceptions—The instructions for power transformers given in Part I 
should be followed for all distribution transformers above 100 Kv-a. in capacity 
or for operation on 22000 volts or higher. 


Location 


Accessibility, ventilation and ease of in- 
spection should be given careful considera- 
tion in locating transformers. 


Self-Cooled Transformers depend en- 
tirely upon the surrounding air for carrying 
away their heat. For this reason, care must 
be taken to -provide adequate ventilation. 
For indoor installation the room in which 
the distribution transformers are placed 
must be well ventilated so that heated air 
can escape readily and be replaced by cool 
air from outside. 


Distribution transformers should always 
be separated from one another and from 
adjacent walls, partitions, etc., in order to 
permit free circulation about the tanks. 
This separation should not be less than 12 in. 


Storage 


When distribution transformers can be set 
up immediately in permanent locations and 
filled with oil, it is advisable to do so even 
though they will not be put into service for 
some time. If this is not convenient they 
should be stored in a dry place having no 
rapid or radical temperature changes, and if 
possible, immersed in dry transformer oil. 
Distribution transformers should not be 
stored or operated in the presence of cor- 
rosive gases, such as chlorine, etc. 


Handling 


Where a distribution transformer cannot 
be handled by a crane, it may be skidded or 
moved on rollers, but care must be taken not 
to damage the base or tip it over. A dis- 
tribution transformer should never be lifted 
or moved by placing jacks or tackle under 
the drain valve or other attachments and 
must not be moved by taking hold of the 
leads. ‘When rollers are used under large 
distribution transformers, skids must be used 
to distribute the stress over the base. 


When working about a transformer 
particular care must be taken in handling all 
tools and other loose articles, since any- 
thing metallic dropped among the windings 
and allowed to remain there may cause a 
breakdown. 


Bo e.c. 


Inspection Preliminary to 
Installation 


Transformers are in first class operating 
condition when shipped by the manufacturer, 
i. e., they have been thoroughly tested for 
defects and are perfectly dry. 


When received, examination should be 
made before removing from cars, and if any 
injury is evident or any indication of rough . 
handling is visible, railroad claim should be 
filed at once and the manufacturer notified. 


Moisture may condense on any material if 
the metal is colder than the air, and if pres- 
ent, it lowers the insulation properties and 
may cause failure of transformer. ‘There- 
fore, if transformers or oil drums are 
brought into a room warmer than they are, 
they should be allowed to stand before open- 
ing until there is no condensation on the 
outside and they are thoroughly dry. 


Before being set up, a transformer should 
be inspected for breakage, injury or mis- 
placement of parts during shipment, and 
thoroughly examined for moisture. All ac- 
cessible nuts, bolts and studs should be 
tightened. 


Pole Mounting—Convenient lugs or eye 
bolts are provided on the side of the case to 
which the rope lifting the transformer may 
be attached. It will be found convenient to 
fasten the hanger irons to the case before the 
transformer is raised to the cross-arm. The 
transformer can then be raised up to and 
slightly above the cross-arm and the hooks 
on the hanger-irons made to engage the 
cross-arm by lowering the transformer. 

The majority of distribution transformers 
are shipped in their tanks without oil. Due 
to the fact that the windings of these trans- 
formers are usually impregnated with a spe- 
cial compound, no drying out of these trans- 
formers is necessary unless the presence of 
moisture is readily apparent from visual in- 
spection. Under such circumstances, drying 
out should be resorted to; otherwise, it is 
unnecessary. If distribution transformers 
are shipped in their tanks filled with oil, the 
oil should be tested for moisture and if 
moisture is present the transformers should 
be dried out. For methods of drying out trans- 
formers refer to instructions for the care 
and operation of power transformers in 
Patel 


April, 1926 


DISTRIBUTION TRANSFORMERS 





Filling Transformers 


Metal hose must be used instead of rubber 
hose, because oil dissolves the sulphur found 
in rubber and may cause trouble by the 
sulphur attacking the copper. 


Pole-mounted transformers may be filled 
with oil, either before or after mounting, as 
desired. It is sometimes necessary to add 
oil a short time after the transformer has 
been installed, due to the fact that the in- 
sulation will absorb a certain amount of oil. 
It may be found necessary to replenish the 
oil from time to time during actual operation 
in order that the normal oil level be kept 
constant. When the transformer oil is re- 
plenished care should be taken that no mois- 
ture finds its way inside the case. 


The oil used should be of the grade rec- 
ommended by the manufacturer, as the suc- 
cessful operation of the transformer de- 
pends upon it to a great extent. 


Replacing the Cover—Great care should 
be exercised in putting on the cover. If the 
gasket is not properly in place or the cover 


13 


not securely bolted to the case, moisture in 
the form of snow or rain may be driven into 
the transformer tank. 


It is very important that the surface of 
the oil when cold (25 deg. C.) be at the 
oil level indicated on the inside of the tank 
or the oil gauge. 


Care of Transformers in 
Service 


The following practice is recommended 
for the care of pole-mounted distribution 
transformers in service: 


(1) The oil level should be inspected once 
every year and enough oil added to 
bring the level up to the mark inside 
the tank or on the oil gauge. 


(2) Every 3 years the oil should be re- 
moved from the tank and replaced with 


clean, dry oil. 


A periodic check of the load should be 
made to make sure a transformer is not 
being overloaded. 


(3) 


April, 1926 





MEMORANDA 


E. P. C. 15 April, 1926 





Transiormer Section 
: of | 
The Electric Power Club 
Cleveland, O., U.S. A. 


MEMBER COMPANIES 


Allis-Chalmers Mfg. Co.._______- Milwaukee, Wis. 
American Transformer Co.________- Newark, N. J. 
Duncan Electric Mfg. Co.________- Lafayette, Ind. 
General Electric Co.________-_ schenectady, N. Y. 
Kuhlman Electric Co._-___-__ _.--Bay City, Mich. 
Moloney Electric Co.-.....-._.-__-_ St. Louis, Mo. 
The Packard Electric Co.__-_...__ Warren, Ohio 
Pittsburgh Transformer Co.______- Pittsburgh, Pa. 
Wagner Electric Corp’n___..____.__St. Louis, Mo. 


Westinghouse Elec. & Mfg. Co._E. Pittsburgh, Pa. 





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